(1) Field of the invention
The present invention relates to an image forming apparatus using an electrophotographic process such as a photocopier, a printer and the like.
(2) Description of the Prior Art
In image forming apparatus using so called electrophotographic process (Carlson process), corona charging devices that utilize the corona discharge phenomenon have been used as typical means for charging an electrophotographic photoconductor at a desired potential level. This method, however, requires a high discharge voltage, which results in electric noises affecting various peripheral apparatus. Alternatively, a large quantity of ozone gas generated in discharge gives an unpleasant feeling to people around the machine. To deal with these problems, as alternatives to corona discharging devices, a method has been proposed in which a photoconductor is charged by applying a voltage between the photoconductor and a conductive resin roller or conductive fibers. Nevertheless, this method suffers from another problem. That is, in a case of a conductive resin roller, if a micro-area of a photoreceptive layer of the photoconductor to be charged is peeled off and therefore part of a conductive substrate such as aluminum, etc., is exposed, electric current from the roller converges into the exposed portion, thereby causing striped charging unevenness extending across the photoconductor in its axial direction. Brush type charging devices using conductive fibers can be roughly classified into two kinds: one has fibers planted on a belt-like strip and the other of which has fibers planted on a roller. Either of these could eliminate striped charging unevenness which arises when the aforementioned conductive resin roller is used.
Nevertheless, when the belt-like brush charging device is used, another kind of image defect arises. Specifically, brushing stripes which run in the advancing direction of sheets arise on the image. This is because each position across the longitudinal direction of the charged member or photoconductor comes into contact with the same part of fibers on the charging brush. That is, if some parts of fibers have less charging ability than other parts, the portion of the charged member contacting with the part of fibers having less charging ability will be charged at a lower surface potential while the portion contacting with the part of fibers having higher charging ability will be charged at a higher surface potential. This causes charging unevenness across the longitudinal direction of the charged member, thereby generating brushing stripes in the advancing direction of sheets. Further, depending on the contacting strengths at contact points between the charging brush and the charged member, the degree of wear to the charging brush and the charged member will differ, that is, some parts will be worn out faster while other parts will not. As a result, charging failure occurs earlier at the portion having been worn out shortening the lives of the brush and the charged member.
To deal with this, it has been disclosed in Japanese Patent Publication Sho 63 No.43749 that the charging brush is vibrated in the direction perpendicular to the moving direction of the charged member. Actual images created as the charging brush is vibrated were found to be free from the brushing stripes running in the advancing direction of sheet which appeared when the brush was fixed. Further, it was confirmed that the lives of the charging member and the charged member were markedly lengthened.
FIGS. 1 and 2 are illustrative views showing configurations of prior art examples. In the figures, A, B, C and D indicate:
A: Length of a charging member; PA1 B: Effective width of a photoconductive layer applied on charged member; PA1 C: Developing width; and PA1 D: Vibrating width of the charging member. PA1 A: Length of a charging member; PA1 C: Developing width; PA1 D: Vibrating width of the charging member; and PA1 E: Length of a cleaning member. PA1 a charged member; and a charging member with conductive fibers, placed in contact with the charged member so as to share at least a contact surface or micro-space between the two members while being vibrated in directions perpendicular to a moving direction thereof wherein a voltage is applied between the charging member and the charged member so as to charge the charged member, and is constructed such that elements are set up so as to satisfy any one or both of the following relations (a) and (b): EQU C+D&lt;A&lt;B-D (a) EQU C&lt;E&lt;A+D (b) PA1 where A denotes a longitudinal width of the charging member; B denotes an effective longitudinal width of a photoconductive layer coated range on the charged member; C denotes a developing width in the longitudinal direction of a developing unit; D denotes a vibrating width of the charging member; and E denotes a longitudinal dimension of a cleaning member for the charged member.
Further, reference numeral 1 designates a photoconductor while numerals 1a and 1b denote a photoconductive layer coated range and a conductor substrate, respectively.
Initially, in the case shown in FIG. 1, where A+D&gt;B, when a charging member 5 is vibrated, the longitudinal extremes of the charging member 5 are made to interfere with the conductive substrate portion 1b on the charged member 1, giving rise to the following problems.
i) Current leak occurs at contacting portions 21 and 22 between the charging member 5 and the conductive substrate 1b, and in consequence, excessive current flows through the charged member 1, causing damage thereto.
ii) In the case where capacity of the power source for the charging device is small or the charging device comes into contact with the conductive substrate 1b in a large area, very few of charges can be supplied to the photoconductive layer portion 1a or the non-conductive portion of the charged member 1, whereby those portions are isolatedly reduced in surface potential causing image defects.
The above problem can be solved when the contact width, i.e., A+D between the charging member and the charged member is set up to be shorter than the effective width B of the charged member. In other words, (the charging member length+the vibrating width) should be smaller than (the effective width of the photoconductive layer applied on charged member) or a relation "A+D&lt;B" should hold.
Next, let us consider the case shown in FIG. 2. When the charging member having a length of A with a vibrating width of D is brought into contact with the charged member to charge it, the width of the range within which the charging member is always in contact with the charged member is (A-D) and therefore only this region can be uniformly charged at a desired surface potential. If the length (A-D) is shorter than the effective developing width C, or C &gt;A-D, the following problems occur.
i) Since edge regions 23 and 24 on the charged member 1 come in contact with the charging member 5 for a shorter time than the middle part of the claimed member 1 and therefore cannot be charged at a sufficiently high surface potential level. Overlapping areas 25 and 26 of regions 23 and 24 overlap with the developing width region C and therefore are toner-developed when development (as performed in laser printers) is executed. As a result, toner debri forms on a transfer member and is wasted. Further, the toner which could not be cleaned up and remains on the charged member may adhere to the charging brush which decrease its charging ability resulting in occurrence of charging unevenness.
ii) Further, since development is always effected in the regions 25 and 26, toner particles, not having been collected efficiently for prolonged use, adhere to a conductive fabric cloth 5a, thereby causing charging unevenness and giving bad influences on resulting images. Further, the developer is consumed rapidly.
This problem can be solved by setting up the width (A-D) of the region which can always be charged at the desired level to be greater than the developing width C. Therefore, a relation "A-D&gt;C" should hold. It should be noted that this requirement can, of course, be applied to the normal development mode which is performed in photocopiers and the like.
Japanese Patent Application Laid-open Hei 3 No.100673 discloses an idea which defines, in an image forming apparatus using a charging member with conductive fibers, dimensional relations as to its charging member width, developing width and charged member width. FIG. 3 illustrates the idea in which the configuration aims at uniform charging of the entire surface of a photoconductive layer as well as extermination of smudge and failure of resulting images. To achieve these purposes, an insulating layer is provided on each extreme of a conductive substrate 1b in order to prevent a charging member 5 from being short-circuited with a charged member 1 while specific limitations are imposed on effective widths of constituting parts. The technique shown in FIG. 3, however, only specifies the length A of the charging member, the effective length B of the charged member and the developing width C so as to satisfy a relation A&gt;B&gt;C. Still, this technique can be applied only to configurations in which the charging member 5 is not vibrated. Accordingly, this technique is quite different from the art now being discussed in question in which the charging member 5 is vibrated, and naturally, the relation among the effective width A, B and C does not include the aforementioned vibrating width D. For this reason, the description of the technique of FIG. 3 is mentioned only for reference and no further discussion on the technique of FIG. 3 will be made.
FIGS. 4 and 5 are illustrative views showing other configurations of a prior art example. In the figures, A, B, C and D indicate:
Initially, in the case shown in FIG. 4, where E&lt;C, the following problems occur.
i) There exist regions 27 and 28 in which it is difficult to collect developing particles not having been transferred and therefore remaining on a charged member 1. This remaining toner adheres to a charging member 5. The thus adhered toner particles are further spread out to wider ranges by the vibrating charging member 5, polluting the image region. Moreover, the adherent particles fix to conductive fiber portions 5a of the charging member 5, thereby likely causing charging defects.
ii) With a charging member 5 made up of conductive fibers 5a, those fibers may detach from the charging member and the detach fibers may adhere to the charged member 1 in the contacting width range between the charging member 5 and charged member 1. Particularly, existence of the detach fibers adhered to places on the charged member near the image region may have an adverse influence on image forming. Hence, removal of the fallen fibers is important. Nevertheless, the aforementioned condition, i.e., E&lt;C, is not enough for removing fibers fallen in regions 27 and 28.
In order to solve the problems above, it is necessary to make the width of the cleaning member wider than, at least, the effective developing width, that is, a relation "E&gt;C" must hold. Therefore, consider the case shown in FIG. 5, wherein a relation "E&gt;A+D" holds. In other words, a cleaning member is provided so as to reach regions 29 and 30 outside the contacting region (A+D) between a charging member 5 and a charged member 1 where very few adherent substances such as developer, fallen conductive fibers and the like exist on the charged member 1. In this case, the following problems occur.
i) In such regions 29 and 30 to which, in practice, only a few adherent substances adhere, frictional force generated between the cleaning member and the charged member 1 tends to become greater, therefore a stronger load torque is required for driving the charged member 1. Further, when the cleaning member is of a blade-type, the blade may be bent backward, and also, this bent blade could damage the charged member 1. Moreover, the cleaning structure becomes enlarged, disadvantageously raising its cost.
To solve the problem, it is necessary to set up the width E of the cleaning member smaller than the contacting width between the charging member 5 and the charged member 1, i.e., a relation "E&lt;A+D" must hold.
Japanese Patent Application. Laid-open Sho 64 No.7070 discloses an idea which defines, in an image forming apparatus in which a charged member 1 is charged by bringing a charging member 5 into contact with the charged member 1, dimensional relations as to its charging member width, developing width and cleaning member width.
This technology originally assumes the use of an organic photo-conductor (OPC) as a charged member 1. Hence, the disclosure exemplified several experimental results for different kinds of OPCs. FIG. 6 is an illustrative view schematically showing a typical configuration of this prior art technology. In this configuration, a relation is defined in which a width E should at least contain a region A.sub.1, where A.sub.1 denotes the region across which a charging member 5 comes in contact with a charged member 1 while E denotes the width of a cleaning member used. Here, the charging member 5 can be selected from those usually used such as of a roller type, a brush-type etc. The reason why the above relation between the region A.sub.1 and the width E of the cleaning member should be defined, that if the small amount of adhered substances existing outside the contacting width between the charging member 5 and the charged member 1 are trapped in regions 31 and 32 between the charging member 5 and the charged member 1, these particles generate pinholes especially when the charged member 1 is made up of those having a low surface hardness such as OPCs. Even if these pinholes exist in areas outside the image region, current leakage occurs when the charging member 5 comes in contact with the pinholes, thus causing adverse effect on resulting images.
The above-described effect is likely to happen or could occur mainly when the charging member 5 used is of a resin roller type or the like, but in the cases shown in FIGS. 1, 2, 4 and 5 in which the charging member 5 used is of a conductive fiber type, generation of pinholes hardly occurs due to adhered substances caught between the charging member 5 and charged member 1. Even the existence of pinholes outside the image region usually does not adversely input resulting images. Further, this disclosure does not have any reference to the configuration of the vibrating charging member 5. Although the aforementioned contacting region A.sub.1 between the charging member 5 and the charged member 1 is to correspond to A+D, (or the charging member length A plus the vibrating width D in the cases shown in FIGS. 1, 2, 4 and 5) it is difficult to compare the configuration shown FIG. 6 equally with those cases since no vibration of the charging member is effected in the configuration of FIG. 6.
To sum up, the following problems occur in systems in which the charging member 5 is brought into contact with the charged member 1 with the charging member 5 being vibrated.
First of all, as concerning the dimensional relation among the charging range width determined by the width of the charging member 5 and its vibrating width, the width of the photoconductive layer coated range 1a on the charged member 1 and the developing width, the following problems occur.
1) In the case where the charging member 5 is in contact with the conductive portion 1b of the charged member 1, excessive current flows through the charging member 5, causing damage to the charging member 5. Alternatively, in the case where the capacity of a power supply for the charger is low or in the case where the charger is in contact with the conductive substrate 1b over a large area, electric charges are not sufficiently supplied to the photoconductive layer portion 1a, or the non-conductive portion of the charged member 1, whereby the portions are isolatedly reduced in surface potential causing image defects.
2) In the case where a region to be charged at a desired surface potential (length of the region corresponds to "the charging member length-the vibrating width") is shorter than the developing width, outer edge portions of the photoconductor corresponding to both extremes of the brush are not brought into contact with the brush for sufficiently long time, so that it is impossible to charge the portions to the desired level. Therefore, as in the reversal developing process adopted as in laser printers etc., the outer edge portions with less surface potential levels always bear toner, causing smudge of the transfer member or waste of toner. Further, the toner which could not be cleaned up may adhere to the charging brush, whereby the charging brush might be deteriorated in its charging ability for prolonged use, causing charging unevenness.
Regarding the dimensional relation among the charging range width determined by the width of the charging member 5 and its vibrating width, the developing width and the length of the cleaning member, the following facts can be pointed out.
1) In order to collect the remaining developer on the charged member 1, it is necessary to make the cleaning member longer than the effective developing width. Further, in the case where the charging member 5 is made up of conductive fibers 5a, the conductive fibers 5a may fall out from the charging member 5 within the contacting width range between the charging member 5 and the charged member 1. Fallen fibers in locations near the image region might adversely influence image. Therefore, the removal of the fallen fibers is very important.
2) If the cleaning member is too long, the frictional force between the cleaning member and the charged member 1 becomes greater in the regions to which, in practice, only a small amount of developer, fallen fibers and the like adhere, therefore, a stronger load torque is required for driving the charged member 1. Further, when the cleaning member is composed of a blade-type member, the blade may be bent backward and could cause damage to the charged member 1. Moreover, the enlarged cleaning structure raises its cost.